Shape cutting machines are used to cut a metal plate into a desired configuration. A cutting torch, such as a gas or plasma arc torch, is mounted to the shape cutting machine vertically above the metal plate, or workpiece. A support bracket connected to the shape cutting machine is responsive to an output drive for moving the torch vertically and laterally to cut the workpiece into the predetermined shape. The support bracket is fixed to a mounting apparatus which supports the torch. The mounting apparatus supports the torch at a working height above the workpiece which results in the optimum cutting efficiency and life of the electrode-nozzle pair.
It is common practice to submerge the workpiece and the lower portion of the torch in a water bath during the cutting operation to reduce the emission of smoke and toxic fumes, to diminish the noise level of the operation, and to allow the arc to be viewed without the use of eyeshields for screening the ultraviolet radiation emitted by the arc. Submerging the workpiece and the lower portion of the torch in a water bath, however, increases the difficulty of setting the working height of the torch above the workpiece.
The efficiency of the cutting operation and the life of the torch, particularly a plasma arc torch, is greatly influenced by the working height. If the torch is too far from the workpiece, the energy of the transferred arc extending from the electrode to the workpiece may not be sufficient to cut the workpiece at the desired rate. If the torch is too close to the workpiece, metal debris from the cutting operation may accumulate on the nozzle and cause a double arc condition which can result in premature destruction of the nozzle-electrode pair. When the torch is at the proper working height, the transferred arc extending from the electrode to the workpiece produces the optimum amount of cutting energy without excessively damaging the electrode-nozzle pair.
Conventional means for setting the working height of the torch above the workpiece include mechanical probes, electrical switches, inductive and capacitive sensors, devices for sensing the gas pressure inside the torch when the nozzle is in close proximity to the workpiece, devices for sensing the pilot arc current as it transfers to the workpiece, and devices for sensing the increased power required by the output drive when the torch contacts the workpiece. Each of these means has a disadvantage, however, particularly when the workpiece and the lower portion of the torch are submerged in a water bath.
Mechanical probes are subject to damage from the water environment, and can be rendered inoperable by the accumulation of metal debris on the moving parts of the probe. Electrical switches are likewise subject to deterioration in the water environment. In addition, the high frequency noise generated when the arc is transferred from the electrode to the workpiece can be conducted through the direct current line from the electrical switch to the programmed control logic which automates the cutting process and commands the output drive. As a result, additional electronics and filters are required to protect the programmed control logic from being damaged by high frequency noise.
Inductive and capacitive sensors suffer from the same shortcomings as mechanical probes and electrical switches because of their proximity to the workpiece and the electrical signals which they generate. Devices which detect changes in the gas pressure inside the nozzle, and devices which sense the pilot arc current as it transfers from the nozzle to the workpiece, cannot be used with torches which are submerged in a water bath. Mounting apparatus which sense the increased power in the output drive once the torch contacts the workpiece may apply excessive force to the lower portion of the torch, or worse, can set the torch at an erroneous working height if the workpiece yields under the force applied to the workpiece by the output drive.
The lower portion of the torch generally includes an insulator which surrounds and protects the electrode and the nozzle inside the torch. Typically, the insulator is made of a material, such as ceramic, which does not thermally or electrically conduct. Ceramic insulators, however, are brittle and can be easily damaged when the force applied to the torch from contact with the workpiece is too great. For example, the ceramic insulator can crack and cause damage to the electrode-nozzle pair inside. Replacement of the insulator, electrode or nozzle usually requires the shaping machine to be shut-down for a period of time which results in a loss of throughput, or production.
An apparatus for mounting a cutting torch in a shape cutting machine above a workpiece submerged in a water bath is disclosed in U.S. Pat. No. 4,415,795 to Ross et al. The patent discloses a mounting apparatus which is adapted to detect contact between the torch and a workpiece in the vertical direction, and between the torch and an obstruction in a lateral direction. The initial working height of the torch is set by lowering a plate-sensing member which is movably mounted to a torch-supporting member until the torch comes in contact with the workpiece. The force applied to the lower portion of the torch when the torch contacts the workpiece is transferred to the plate-sensing member. Upward movement of the plate-sensing member relative to the torch-supporting member opens an electrical switch which sends a signal to the programmed control logic to command the output drive to reverse. The output drive is reversed for a predetermined time at a predetermined rate to obtain the proper working height of the torch above the workpiece.
A disadvantage of the mounting apparatus disclosed in the patent to Ross et al. is that a force greater than the weight of the torch must be applied to the lower portion of the torch before the mounting apparatus detects contact between the torch and the workpiece. Specifically, a force sufficient to overcome the frictional resistance of the plate-sensing member, and cause movement of the plate-sensing member relative to the torch-supporting member, is required to open the electrical switch which signals the programmed control logic to command the output drive to reverse. As a result, significant damage to the lower portion of the torch can occur.
The lower portion of the torch disclosed in the patent to Ross et al. is fitted with evenly-spaced metallic protrusions which extend outwardly from the bottom of the torch to protect the torch from the contact with the workpiece. The protrusions, however, cannot protect the torch if the workpiece includes imperfections on its upper surface. For example, oftentimes molten products from the cutting process will accumulate on the surface, cool, and form small, dense nodules which are higher than the depth of the protrusions. When the torch contacts the workpiece, the force applied to the lower portion of the torch is concentrated over a small surface area on the bottom of the torch opposite the nodule. The resulting local stress can exceed 100 pounds per square inch and can easily damage the ceramic insulator.
Another disadvantage of the mounting apparatus disclosed in the patent to Ross et al. which is equally, if not more important, is that the force applied to the workpiece by the lower portion of the torch can cause the working height to be set erroneously. If the workpiece is thin, or is made of a soft metal, the plate may flex downwardly under the force applied to the workpiece by the lower portion of the torch before the output drive reverses. After the output drive reverses, the plate returns to its unloaded, relaxed position. The top surface of the workpiece is then nearer to the bottom of the torch than desired, and the actual height of the torch above the workpiece is less than the desired working height.
On the other hand, if one of the protrusions extending from the bottom of the torch encounters a nodule, as described above, on the surface of the workpiece, the concentration of stress can cause the motor to reverse prematurely. In that case, the height of the torch above the workpiece will be greater than the desired working height.
Another disadvantage of the mounting apparatus disclosed in the patent to Ross et al. is that the electrical switches which signal the output drive to reverse may deteriorate and fail in the water bath environment. Failure of the electrical switches can result in severe damage to the ceramic insulator and the electrode-nozzle pair inside the lower portion of the torch. Further, there is no provision on the mounting apparatus of the patent to Ross et al. to accurately return the torch to the same operating position after the torch is removed from the apparatus.
U.S. Pat. No. 5,160,821 to Oakley and assigned to the assignee of the present invention, also discloses an apparatus for mounting a plasma arc torch to a shape cutting machine. The mounting apparatus of the patent to Oakley, however, positions the torch at the proper working height above the workpiece without the use of a mechanism which is subject to friction, or electrical switches which may deteriorate in the water bath environment.
Instead, the mounting apparatus includes a torch-supporting plate and a movement-sensing plate which are resiliently connected together. Three rigid balls are arranged in a triangular pattern between the plates with each ball seated in a concave ball seat of a pressure release valve assembly formed in one of the plates. When the lower portion of the torch comes in contact with the workpiece or an obstruction, the plates move relative to one another and at least one of the three balls is displaced from its ball seat.
When a ball is displaced from its ball seat, pressurized gas is released from the pressure release valve assembly associated with the displaced ball, thus producing a pressure differential in a gas line between the torch and the cutting machine. The pressure difference is sensed by an actuator which causes an electrical switch, located remote from the water bath environment, to signal the programmed control logic to reverse the output drive so that the torch moves away from the workpiece or the obstruction. After the force applied to the torch is removed, the resilient connection between the plates forces the balls, and thus the torch, to return to the same vertical location and angular orientation as before the contact.
While the mounting apparatus disclosed in the patent to Oakley is satisfactory in many respects, it too requires a substantial amount of force to be applied to the lower portion of the torch before the apparatus detects contact between the torch and the workpiece. Specifically, the force applied to the lower portion of the torch must be sufficient to overcome the resistance of the resilient mounting between the plates, and thus can result in damage to the sensitive tip of the torch.
Accordingly, it is an object of the invention to provide an apparatus for mounting a cutting torch to a shape cutting machine which is adapted to detect contact between the torch and a workpiece in the vertical direction, and between the torch and an obstruction in any lateral direction.
It is a more particular object of the invention to provide a mounting apparatus for setting the working height of a plasma arc torch above a workpiece submerged in a water bath which reduces the likelihood of damage to the lower portion of the torch and the electrode-nozzle pair.
It is another object of the invention to provide a mounting apparatus of a cutting torch including a mechanism for setting the working height of the torch which reduces the amount of force applied to the lower portion of the torch when the torch contacts a workpiece or an obstruction.
It is another object of the invention to provide a mounting apparatus for a cutting torch including a mechanism for setting the working height of the torch which does not include an electrical switch which may deteriorate in the operating environment of the torch, or which allows electrical noise to interfere with the programmed control logic of the torch.
It is another object of the invention to provide a mounting apparatus for a cutting torch including a mechanism for setting the working height of the torch which does not require a relatively long response time.
It is another object of the invention to provide a mounting apparatus for a cutting torch which returns the torch to the same operating position after the torch is removed.
It is another object of the invention to provide a mounting apparatus for a cutting torch which returns the torch to the same vertical location and orientation after the torch contacts an obstruction in a lateral direction.